Growth of large and uniform semiconductor substrates, using III-V semiconductor substrates such as GaAs, are now routinely required for electronic and optoelectronic applications. Currently, semi-insulating semiconductor substrates for integrated circuit devices and substrates for thin film growth are usually prepared by the liquid encapsulated Czochralski ("LEC") technique and the horizontal Bridgman ("HB") technique, respectively. Czochralski first proposed his method of crystal growth in Zeits. Phys. Chem. 92 (1918) 219, and the method was first used for growth of germanium single crystals in 1950. R. Gremmelmeier, Zeits. Naturforschung, 11A (1956) 511. Metz et al., Jour. Appl. Phys. 33 (1962) 2016, and later Mullin et al., Jour. Phys. Chem. Solids 26 (1965) 782, developed a liquid encapsulation technique that could be applied to the Czochralski method. The LEC technique became the industry's standard for production of semi-insulating semiconductor crystals such as GaAs, especially after it was demonstrated that pyrolytic boron nitride ("PBN") crucibles could be used to produce undoped, semi-insulating crystals.
The HB technique and a horizontal gradient freeze ("HGF") technique were widely used to grow doped conducting semiconductor crystals as well as chromium-doped semi-insulating crystals. It was assumed that the horizontal configuration was necessary in order to limit confinement of the crystal and to allow free expansion of the crystal as it solidified. This technique is still widely used mainly to produce doped crystals because preventing silicon contamination from the quartz ampule is difficult.
A vertical Bridgman ("VB") technique and a vertical gradient freeze ("VGF") technique were discussed by Fisher, Jour. Electrochem. Soc., 117 (1970) 41C, in a review of crystal growth techniques for II-VI and III-V compounds. Three years later, Blum et al., Jour. Electrochem. Soc. 120 (1973) 588, reported successful growth of 1.5 cm diameter GaP single crystals in PBN crucibles by a liquid encapsulated vertical freeze technique ("LE-VGF"). These crystals were found to be appreciably less strained, lower in dislocation density, easier to grow, and with improved diameter control, as compared to the conventional LEC growth technique for GaP. However, when the LE-VGF technique was scaled up to grow larger diameter GaP crystals, spurious nucleation occurred at the crystal/seed melt junction, and reproducibility in growing single crystals was lost, as reported by Woodbury, Jour. of Crystal Growth 35 (1976) 49. Chang et al., Jour. of Crystal Growth 22 (1974) 247, have demonstrated the flexibility of the VGF technique in growing small diameter GaAs unseeded ingots. In 1986 Gault et al., Jour. of Crystal Growth 74 (1986) 491, first studied the growth of high quality III-V semiconductor crystals of large diameter. U.S. Pat. No. 4,404,172, issued to Gault, discloses use of a VGF technique for GaAs crystal growth.
However, the problem of use of the LE-VGF technique or a liquid encapsulated VB technique for growth of large diameter single crystals still faces the problems of random nucleation within the crucible during crystal growth, high dislocation densities and general difficulty of control. Hoshikawa et al., Jour. of Crystal Growth 94 (1989) 643, have used the VGF method with liquid encapsulation provided by boron oxide that is placed at the top of the semiconductor charge in the crucible and allowed to melt and completely cover the top surface of the semiconductor charge during crystal growth. This technique may be called the liquid encapsulated, vertical Bridgman ("LE-VB") technique and does not require arsenic pressure control. A PBN crucible about 8 cm in diameter was used for crystal growth, and vertical growth rate is approximately 0.6 cm/hour. Dislocation density within the whole crystal was not uniform and varied from 5,000 to 40,000/cm.sup.2. A relatively low temperature gradient of 15.degree.-40,000.degree. C./cm near the crystal growth interface was maintained in order to control crystal diameter variation.
Another approach for control of random nucleation and "sticking" at the crucible side walls is disclosed by Clemans et al. in U.S. Pat. No. 4,923,561. Here, the side walls of a PBN crucible are first oxidized to an unstated depth, using formation of native oxide thereon, and a GaAs single crystal is then grown in the crucible by the VGF technique. No liquid encapsulation of the entire semiconductor charge is used here. No dislocation density numbers are reported, nor is the problem of crystal diameter control discussed.
What is needed is a technique for growing III-V and II-VI semiconductor single crystals that will suppress random nucleation and sticking at the crucible walls, will produce low dislocation densities, will allow control of crystal diameter and will be reasonably easy to use.